The invention relates to an XRF (XRF=x-ray fluorescence) measurement apparatus, comprising                an x-ray source for generating x-rays,        x-ray optics for directing x-rays from the x-ray source to a sample,        the sample,        and an EDS (EDS=energy dispersive spectroscopy) detector for detecting fluorescent x-rays from the sample.        
Such an XRF measurement apparatus is known from U.S. Pat. No. 5,778,039 A.
Wafers, in particular silicon wafers, are a basic component in the production of semiconductor electronics. These semiconductor electronics are based on pn-transitions, in particular in diodes and transistors. Semiconductor material of p-type and n-type is produced by carefully controlling the chemical composition of a basic material (such as silicon). More specifically, dopant materials having a number of valence electrons different from the basic material are deliberately added to the basic material.
However, contaminations may act similar to dopant materials, changing the properties of the semiconductor material in an unintended way. Accordingly, semiconductor production is performed under clean room conditions, and the contamination levels are monitored closely.
For silicon wafers, it has been proposed to examine the flat side surfaces of the wafer by means of TXRF (total reflection x-ray fluorescence) spectroscopy. In TXRF, a typically monochromatic x-ray beam is directed to a sample surface, and characteristic x-rays resulting from the refilling of depleted deep electron shells of the sample material are detected. Contaminations of the sample surface result in x-ray peaks at additional wavelengths, as compared to the sample material alone. XRF spectra may be evaluated quantitatively, for determining the amount of contaminations. The flat side surface may be completely scanned with the x-ray beam (“wafer mapping”), if desired.
During production processes, wafers have to be transported at numerous occasions. For this purpose, grippers typically act on the bevel of the wafer; the bevel is also sometimes called “grip edge”. Thus contaminations of the flat side surfaces of the wafer shall be avoided.
However, contaminations of the bevel may be passed on to the flat surfaces later on, for example by surface diffusion, in particular at elevated temperatures. Therefore, bevel contaminations should be avoided, too, and accordingly, bevel contaminations should be monitored for this purpose.
For monitoring bevel contaminations, it is possible to wipe the edge of the wafer with a receptive carrier (such as a cotton bud), and to analyze the receptive carrier, for example with ICP-MS (inductively coupled plasma mass spectrometry). However, this is a complex and time-consuming procedure, and the receptive carrier itself may contaminate the wafer.
It is the object of the invention to allow an improved contamination control of wafers, in particular silicon wafers.